The invention relates to a swivel device for use in a ship equipped for carrying out production/ drilling, etc., in connection with submarine oil/gas wells, said ship being formed with a vertically through-going shaft accommodating a turret rotatably mounted to the ship's hull around a vertical rotational axis, about which the ship may turn when the turret is moored to the seabed by means of anchor lines, preferably in the form of mooring chains.
Through the shaft which is surrounded by the turret, it is possible to lower down and draw up a riser production tubing string, and at a lowermost shaft portion widened through a conical transition portion it is possible to accommodate a so-called STL-buoy which is an immersed buoyancy body moored to the seabed and having transition pieces for risers each extending to a well head.
In the mooring arrangement of the turret, at least one mooring chain might be included, said mooring chain having a fixed length and, for the attachment of its upper end to the turret, is assigned an attachment hook, the other chains included in the mooring arrangement having changeable length and each being assigned thereto a guide pulley at the level of the hook(s) and, preferably, also an individual winch disposed at a higher level than the attachment hook(s) as well as an individual locking means for locking the respective, longitudinally changeable mooring chain in stretched, tensioned, shortened condition.
The turret may, in addition to its rotatable mounting around a vertical axis, be raisable/lowerable for temporarily uncovering and making available underlying structure, e.g. said attachment hooks for some of the mooring chains.
The turret's mounting device for the relative rotatability between ship and turret may, intermediate ship's hull and turret, comprise an upper combined roller-ball bearing and a lower roller bearing, each roller having a vertical axis.
Alternatively to a physical mooring to the seabed, the ship may be positioned on the field through so-called dynamic positioning as previously known.